This invention relates to pellet mills used to form pellets from a flowable stream of material such as slurries and viscous liquids and/or solids to form them into three-dimensional articles of a defined shape by compressing and pelleting the material to be processed. This may include plastics, minerals, waste, agricultural products and animal feed, among other things. The pelleting process yields products that are relatively easy to handle, store, transport and use.
The flowable stream of material to be pelletized may vary widely in its characteristics with regard to relative hardness/ability to abrade and corrosivity.
The pelleting device is a perforated layer of metal, called a die, which may be of any desired configuration such as a flat steel plate or, in the case of the present invention in its preferred aspect, a cylindrical die having a plurality of holes through which the flowable stream of material is passed and pelletized. Mechanical energy is supplied to the flowable stream of material and this energy, in turn, forces the material through the die where it attains a pellet-like shape and is exited from the die. The pelleting process generates significant stresses in the pellet mill die such that physical design factors of the die directly relate to the capacity of the material from which the die is formed to withstand the challenge of the pelleting process which in addition to pressure includes potentially corrosive materials and highly abrasive substances. The flowable stream of material is mechanically pressed against the die surface and pushed through the die as it is converted from a flowable stream of material to a three-dimensionally shaped pellet.
The working surface of the die, that is the surface of the die exposed directly to contact with the mechanical means forcing the flowable stream of material through the die, the die holes themselves and the opposite side of the die from the working surface are subject tc mechanical and sometimes corrosive and abrasive forces. We have surprisingly found that forming the die from a hot-work tool steel arid nitriding the working surface of the die not only provides a more robust, longer-lasting die under conditions of high pressure, temperature and abrasion but also have observed an increased throughput through the die, and an increase in the production rate that was not expected.
Preferably the material from which the die is fabricated is a chromium hot-work steel in the type H10, up to and including the H19 range. We have found that these chromium hot-work steels have good resistance to heat softening owing to their chromium content and include carbide-forming elements. This resistance to heat softening permits the heat treat formation of a nitrided surface on the die while substantially maintaining high core hardness.
Preferred is H13, a "Hot-Work" tool steel originally developed for use in manufacturing tooling for the metal-forming industry. H13 is one of several steels of the H-group of hot-work tool steels. This group of steels was developed to resist softening at hot metal working service temperatures of 900.degree. to 1400.degree. F. (480.degree. to 760.degree. C.) and are commonly used in manufacturing hot-metal-forging dies, hot-metal-extrusion dies, cold-metal-forming dies, and metal-shearing knives. Nitriding is sometimes used in treating H13 hot-metal-extrusion dies to increase service life. This invention includes the use of all H-group steels as cylindrical feed pelleting die steel in the case-hardened (nitrided, carburized, nitrocarburized, etc.) condition.
The ability of these hot-working steels to resist softening at elevated temperatures, for instance, in the case hardening range of 900.degree. to 1400.degree. F., (480.degree. to 760.degree. C.) means they are adapted to substantially maintaining high core hardness during surface heat treatment procedures. In particular gas nitriding is preferred as a case-hardening process in which nitrogen is introduced into the surface of the steel. This process results in an increased surface hardness, improved resistance to wear, improved fatigue life and improved corrosion resistance. The hot-work tool steel nitrided surface pelleting dies of the present invention are able to maintain core hardness values above 50 HRC. In some instances, the resistant nitrided surface layer is greater than 67 HRC while the core of H13 tool steel has a value of 55 HRC.
A particular design, configuration and physical arrangement of the dies vary widely depending upon the material being processed. The number of holes in a given surface area (hole count) and patterning which the holes are displayed will vary accordingly. Preferably the holes include a "relief" area which allow the operator to exercise control over pellet size and throughput.
In a preferred embodiment according to the present invention, there is provided a pellet mill die construction for pelleting particulate material, comprising a pelleting die having first and second surfaces on opposite sides of the die and a plurality of die holes through the die extending between the first and second surfaces for passing the particulate material between the first surface and the second surface, the die being formed of H-group steel, with one of the first surface, the second surface and the plurality of die holes having a nitrided surface layer for wear resistance.
In a further preferred embodiment according to the present invention, there is provided a pelleting mill comprising a particulate material feeder, an inlet for supplying particulate material to the feeder, a cylindrical pelleting die for receiving the particulate material from the feeder, the pelleting die having inner and outer surfaces and a plurality of die holes through the cylindrical die extending between the inner and outer surfaces for passing the particular material from the inner surface through the holes to the outer surface, one of the inner surface, the outer surface and the plurality of holes having a nitrided surface layer formed on an H-group alloy for wear resistance and an outlet for discharging the pellets from the pellet mill.